1. Field of the Invention
The present invention relates to an optical information recording medium, which can record and reproduce information by using laser light. The invention particularly relates to a heat-mode type optical information recording medium which contains an azo dye having a specific structure and is suitable for recording information with laser light.
2. Description of the Related Art
Optical information recording media (optical disks) capable of recording information only once with laser light have been known. These optical disks are also called write-once CDs (so-called CD-Rs) and are typically structured to have a recording layer made of an organic dye, a light reflecting layer made of a metal such as gold, and a protective layer made of a resin. These are laminated in this order on a transparent disk substrate. Information is recorded by irradiating the CD-R with near-infrared laser light (usually laser light having a wavelength in the vicinity of 780 nm). The irradiated portions of the recording layer absorb the light, and the temperature in those portions rises, causing a physical or chemical change (for example, generation of pits). This changes the optical characteristics of the irradiated portions, and information is thus recorded. The reading (i.e., reproduction) of information is carried out by irradiating the CD-R with laser light having the same wavelength as the laser light used for recording, and the information is reproduced by detecting the difference between reflectance in the recorded portions where the optical characteristics have been changed and the unrecorded portions where the optical characteristics have not been changed in the recording layer.
In recent years, there has been a need for an optical information recording medium having a higher recording density. To meet this demand, optical disks called write once digital versatile disks (so-called DVD-Rs) have become commercially available. DVD-Rs have a structure in which two disks are bonded to each other with an adhesive. These disks are usually obtained by disposing a recording layer made of a dye, a light reflecting layer, and, if necessary, a protective layer in this order on a transparent disk substrate having a guide groove (pre-groove) for laser light irradiated to track having a width (0.74 to 0.8 μm) of less than half of the width of the guide groove of a CD-R. The disks are bonded to each other such that the recording layers face inside. Alternatively, DVD-Rs have another structure in which the disk and a disk-shaped protective substrate having the same shape as the disk are bonded to each other with an adhesive such that the recording layer faces inside. Information is recorded and reproduced by irradiating the DVD-R with visible light (usually laser light having a wavelength range of 630 nm to 680 nm). DVD-Rs make it possible to record at higher densities than CD-Rs.
In recent years, networks such as the Internet and high-vision TV have been spreading rapidly. There has also been an increased demand for large capacity recording media that can record image information simply at low cost with High Definition Television (HDTV) broadcasting near at hand. The DVD-R satisfies the demand for a large capacity recording medium to some extent, however, it does not have enough recording capacity to cope with future needs. For this reason, optical disks are being developed that have improved recording density by using laser light having shorter wavelengths than the laser light used for DVD-Rs. This endows the disks with larger recording capacities. For example, an optical recording disk called a Blue-ray system using a 405 nm blue laser light has been placed on the market.
A recording and reproducing method is disclosed in which an optical information recording medium provided with a recording layer having an organic dye is irradiated from the recording layer side to the optical reflecting layer side with laser light having a wavelength of 530 nm or less to thereby record and reproduce information. Specifically, an information recording and reproducing method has been proposed in which an optical disk using a porphyrin compound, an azo dye, a metal azo dye, a quinophthalone dye, a trimethinecyanine dye, a dicyanovinylphenyl skeleton dye, a cumarin compound, or a naphthalocyanine compound as the dye of the recording layer is irradiated with blue (wavelength: 400 to 430 nm or 488 nm) or bluish green (wavelength: 515 nm) laser light to record and reproduce information.
Various dyes have been proposed as the dye for these optical disks using a blue light-emitting laser (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2001-287460, 2001-287465, 2001-253171, 2001-39034, 2000-318313, 2000-318312, 2000-280621, 2000-280620, 2000-263939, 2000-222772, 2000-222771, 2000-218940, 2000-158818, 2000-149320, 2000-108513, 2000-113504, 2002-301870 and 2001-287465, and U.S. Patent Laid-Open No. 2002/0076648A1).
Nonetheless, the inventors of the present invention have discovered that optical disks using the known dyes described in the above Patent References require further improvement. This is due to the fact that these disks do not reach the level at which recording characteristics such as reflectance and degree of modulation are satisfactory and also, these disks have unsatisfactory storage stability.
In the meantime, in order to produce an optical disk at low cost, it is preferable to use a production method in which a dye is dissolved in a solvent to prepare ink (i.e., coating solution), and the ink is then applied to a substrate by spin coating. However, when the dye used for an optical disk has insufficient solubility or dissolution stability in the solvent, it is impossible to produce an optical disk using the dye with this production method including coating. Also, this method has problems concerning the coatability of the dye and drying load when producing an optical disk. The performance of an optical disk is largely dependent on the structure and properties of the dye. The dye necessary to read and write information with light having a wavelength of 450 nm or less must have absorption in the ultraviolet region and a large refractive index at the wavelength of writing light. However, many such dyes have a low molar extinction coefficient and low solubility in a solvent for coating solutions, and none of these dyes satisfy requirements for optical disks. If the structure of the dye is modified to introduce a substituent improving volubility in a dye molecule, the molecular weight of the dye increases, which resultantly drops the refractive index of the dye film. Conventionally, it is difficult to satisfy the demand for both high solubility and high refractive index.
Also, many dyes having absorption in the ultraviolet region have low fastness to light, humidity, temperature and atmosphere. If the fastness can be improved, dyes that have high molar extinction coefficients but have not been put to practical use because of their low fastness can be put to practical use.
Therefore, it is necessary to develop an optical information recording medium that has excellent recording characteristics and can record and reproduce information at high density by laser irradiation, particularly with laser light having a wavelength of 450 nm or less.
Also, these is a need for an optical information recording medium that is stable in high-temperature and high-humidity conditions, and can store recorded information for a long period of time.
Moreover, there is a need for an optical information recording medium having improved dye solubility, coatability, and drying load which are typically problematic when producing disks.